To satisfy demands for wireless data traffic having increased since commercialization of 4th generation (4G) communication systems, efforts have been made to develop improved 5th generation (5G) communication systems or pre-5G communication systems. For this reason, the 5G communication system or the pre-5G communication system is also called a beyond-4G-network communication system or a post-long term evolution (LTE) system.
To achieve a high data rate, implementation of the 5G communication system in an ultra-high frequency (mmWave) band (e.g., a 60 GHz band) is under consideration. In the 5G communication system, beamforming, massive multi-input multi-output (MIMO), full dimensional MIMO (FD-MIMO), an array antenna, analog beamforming, and large-scale antenna technologies have been discussed to alleviate a propagation path loss and to increase a propagation distance in the ultra-high frequency band.
For system network improvement, in the 5G communication system, techniques such as an evolved small cell, an advanced small cell, a cloud radio access network (RAN), an ultra-dense network, a device to device (D2D) communication, a wireless backhaul, a moving network, cooperative communication, coordinated multi-points (CoMPs), and interference cancellation have been developed.
In the 5G system, advanced coding modulation (ACM) schemes including hybrid frequency-shift keying (FSK) and quadrature amplitude modulation (QAM) modulation (FQAM) and sliding window superposition coding (SWSC), and advanced access schemes including filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) have been developed.
NOMA is a technique for allowing a plurality of user equipments (UEs) to use identical non-orthogonal time/frequency resources, thereby improving the performance of a system and the fairness of scheduling of the UEs. When compared to NOMA, orthogonal frequency divisional multiple access (OFDMA) used in a general communication system may be referred to as orthogonal multiple access (OMA).
In a NOMA system, an evolved NodeB (eNB) allocates identical time/frequency resources to multiple UEs, and transmits downlink (DL) signals for respective UEs in a superposed manner. Each UE cancels a signal of another UE from its received signal (successive interference cancellation) to restore its signal.
FIGS. 1A, 1B, and 1C illustrates an example to compare communication schemes between an eNB and a UE between a NOMA system and an OMA system according to the related art.
Referring to FIGS. 1A to 1C, it is assumed that when identical time/frequency resources are allocated to two UEs, UE1 103 and UE2 105, an eNB 101 allocates low power (e.g., ⅕ of available power) to a signal 1 for the UE1 103 that is the nearer UE located closer to the eNB 101 than the UE2 105, and allocates high power (e.g., ⅘ of the available power) 133 to a signal 2 for the terminal UE2 105 that is the far UE located farther from the eNB 101 than the UE1 103, and transmits the two signals to the UE1 103 and the UE2 105 in a superposed manner.
When the UE1 103, which is the near UE, receives the superposed signal, attenuation of the signal 1 and the signal 2 may not be large because the UE1 103 is located close to the eNB 101. Herein, since a component of the signal 2 may occupy a large part of the superposed signal received by the UE1 103, a signal-to-noise and interference ratio (SINR) of the superposed signal received by the UE 1 103 is relatively high. Referring to FIGS. 1A to 1C, for example, an SINR is expressed as 20 dB.
On the other hand, if the UE2 105, which is the far UE, receives the superposed signal, the power strength of the signal 1 is low and the UE2 105 is located far from the eNB 101, such that the signal 1 may arrive at the UE2 103 after being mostly attenuated. Thus, the superposed signal received by the UE2 103 may include a component of the signal 1 with power of a level that is similar to noise. As a result, most components of the superposed signal received by the UE2 105 may be occupied by the signal 2. Moreover, the signal 2 may be attenuated while arriving at the UE2 105, such that the magnitude of the SINR of the signal received by the UE2 105 is small. Referring to FIGS. 1A to 1C, for example, an SINR is expressed as 0 dB.
In this example, the superposed signal received by the UE1 103 includes the signal 1 and the signal 2, and the power of the signal 2 is much higher than that of the signal 1, such that the UE1 103 may easily distinguish the signal 1 from the signal 2. Thus, the UE2 cancels the interference of the signal 2 from the superposed signal and receives the signal 1. Meanwhile, in the signal received by the UE2 105, the signal 1 may be processed as noise and only the signal 2 may be processed as a signal component, such that the UE2 105 may receive the signal 2 without a need to perform interference cancellation with respect to the signal 1.
FIG. 1B shows system performance in OMA such as OFDMA. That is, FIG. 1B shows DL transmission performance for the UE1 103 and the UE2 105 when ½ of an available bandwidth 121 and full available power 123 are allocated to the UE1 103 and the UE2 105, respectively. A data rate R1 of the UE1 103 is 3.33 bps/Hz, and a DL data rate R2 of the UE2 105 is 0.50 bps/Hz. Thus, in case of OFDMA, the transmission speed of the entire system is 3.83 bps/Hz.
FIG. 1C shows system performance in a NOMA scheme. When a full available bandwidth is allocated to the UE1 103 and the UE2 105 and ⅕ of available power, 131, and ⅘ of the available power, 133, are allocated to the UE1 103 and the UE2 105, respectively, the DL data rate R1 of the UE1 103 is 4.39 bps/Hz and the DL data rate R2 of the UE2 105 is 0.74 bps/Hz. Thus, in case of NOMA, the transmission speed of the entire system is 5.11 bps/Hz. As such, it can be seen that the entire system performance of NOMA is better than the entire system performance of OFDMA.
In the NOMA system, the overall system performance gain increases as an SINR difference between DL superposed signals received by the UEs that share resources increases. Thus, in the NOMA system, the eNB selects UEs having a large SINR difference between received signals and allocates an identical resource to the UEs. Although it is assumed that two UEs share a resource in FIGS. 1A to 1C, the number of UEs sharing an identical resource may be greater than or equal to 2. However, it is known that a performance gain is not large for three or more UEs even if the number of UEs sharing an identical resource increases.
Meanwhile, the 3rd generation partnership project (3GPP) LTE Rel-13 is discussing application for NOMA to the LTE system. However, to support NOMA in LTE, a detailed scheme for transmitting DL information has not yet been determined.
In particular, in the LTE standard specifications, UEs may have different transmission modes (TMs) and different precoding information. If an identical resource is allocated to UEs having different precoding information and/or different TMs, a performance gain increases largely. Therefore, there is a need for a scheme in which UEs having different TMs and/or different precoding information share resources by using NOMA in the LTE communication system.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.